Summary The aircraft, with an instructor pilot and private pilot on board, departed Ottawa, Ontario, on a planned night cross-country training flight to Cornwall,, Mirabel, Quebec, and return to Ottawa. During the climb to cruise flight, the smell of hot oil was detected in the cockpit. The engine oil temperature was observed to be higher than normal, but the oil pressure was normal. The smell was attributed to possible spillage during the addition of oil to the aircraft engine (Bombardier-Rotax GMBH Type 912A3) before departure. The smell faded away and did not reoccur, so the flight was continued. After arriving at Cornwall, a touch-and-go landing was completed on runway 28. At about 300 feet above ground level (agl), the after take-off checks were completed and the engine oil temperature was again noted to be higher than normal, but now the oil pressure was observed to be lower than normal. At about 500 feet agl, a decision was made to return to the airport and land on the reciprocal runway, runway 10. The private pilot completed the turn back to the airport; however, being close to the runway the aircraft was high on the approach. The private pilot placed the aircraft into a forward slip to quickly lose altitude, but the instructor determined that the aircraft could not be landed on the remaining runway without experiencing a runway excursion. The instructor pilot took control of the aircraft and initiated an overshoot at low level. Shortly thereafter the aircraft engine lost all power, and the instructor pilot carried out a forced landing into a field adjacent to the airport. During the landing, the nose gear was torn from the aircraft causing the aircraft to flip over. The crew sustained minor injuries, and the aircraft was substantially damaged. Ce rapport est galement disponible en franais. Other Factual Information The instructor pilot and private pilot were qualified and certified for the flight in accordance with existing regulations. The private pilot was receiving dual instruction toward his night rating when the accident occurred. The aircraft was examined and no abnormalities were noted except with the aircraft engine. During the examination of the engine it was observed that a clamp installed on the No. 2 exhaust pipe to secure asbestos tape to the pipe had been chafing the outboard end of the pressure oil filter. This chafing action, sustained by the vibration of the engine during operation, had worn a hole in the filter case that allowed oil to escape from the engine which resulted in diminished oil pressure. The No. 2 exhaust pipe is fastened to the No. 2 cylinder head on studs by M8 self-locking nuts (Rotax part number (P/N) 942-035). It was discovered that these nuts had lost their torque and became loose which allowed the exhaust pipe to become loose, causing contact with the oil filter. Oil had sprayed over the windshield of the aircraft. The exhaust nuts on Nos. 1 and 4 cylinders were also found loose but the exhaust nuts on the No. 3 cylinder were tight. Review of the aircraft maintenance records revealed that work had been done on cylinder Nos. 1, 2, and 4 that involved the removal and installation of the exhaust nuts. The locking mechanism of the M8 lock nut is an oval-shaped thread hole manufactured into the nut cylinder which, on installation on the exhaust stud, pinches the stud thread. During removal of the lock nut, the pinch on the stud thread is reduced and the locking capability is diminished. The aircraft maintenance manual states that self locking nuts must be replaced with new items after removal in the event that the friction torque has diminished. Testing conducted at the Transportation Safety Board of Canada, Engineering Branch, determined that each time a M8 lock nut is installed and removed from an exhaust stud on the ROTAX engine, the friction torque diminishes. Investigation also determined that the exhaust nuts on the occurrence engine had been re-installed and that the maintenance facility had no M8 self-locking exhaust nuts in their spares stock. A spring contained in the original oil pressure filter (Rotax P/N 825-700) was causing cracks to develop in the end of the filter housing. Diamond Aircraft recommended the replacement of this assembly in Diamond Aircraft Alert Service Bulletin No. DA20-79-04A, Rev. 0, issued 10 January 1997, with oil filter Rotax P/N 825-701. The replacement filter is dimensionally longer than the original filter and, when installed on the engine, reduces the end clearance with the No.2 exhaust pipe. Further, each time the filter is required to be replaced, the No. 2 exhaust pipe must be removed and reinstalled.